Ruby and Sapphire

Stuff

Effort values are special values in Ruby and Sapphire. Each and every pokémon you battle gives the pokémon that faught it a certain ammount of effort points in certain stats. For example, if you battle a Sceptile, you will get 3 effort points in Speed. The effort points are not shared and your pokémon gets just as many effort points if you train it with the Exp.share as if it battles. A pokémon can only gain a total of 510 Effort points, and only 255 Effort points can go into one stat. That means if you max out two stats, the other four will be at minimum. (This explains why Ruby and Sapphire pokémon have such high stats...) So choose wisely what stats you want effort points in, don't waste your time battling Whismur with Shedinja because they give HP effort points and Shedinja can never have more than one HP. Check tips and tricks for tips on where to train for certain effort points. Four effort points raise a stat by one and it's rounded down, so if you have 255 Effort points in a stat, three of them are wasted (you get 63 extra stat points, same as if you had 252 effort points). So if you want to max out two stats, give them 252 effort points and put the six precious remaining effort points into a third stat to increase it by one.

Natures

Natures increase one stat and decrease another, or they can have no effect. Here I will list all the natures and what they do.

Hardy, Docile, Serious, Bashful and Quirky are neutral, they have no effect on your pokémon.

Lonely increases Attack and decreases Defense.

Brave increases Attack and decreases Speed.

Adamant increases Attack and decreases Special attack.

Naughty increases Attack and decreases Special defense.

Bold increases Defense and decreases Attack.

Relaxed increases Defense and decreases Speed.

Impish increases Defense and decreases Special attack.

Lax increases Defense and decreases Special defense.

Timid increases Speed and decreases Attack.

Hasty increases Speed and decreases Defense.

Jolly increases Speed and decreases Special attack.

Naive increases Speed and decreases Special defense.

Modest increases Special attack and decreases Attack.

Mild increases Special attack and decreases Defense.

Quiet increases Special attack and decreases Speed.

Rash increases Special attack and decreases Special defense.

Calm increases Special defense and decreases Attack.

Gentle increases Special defense and decreases Defense.

Sassy increases Special defense and decreases Speed.

Careful increases Special defense and decreases Special attack.

Now, why would you want to raise a stat at the cost of another? You may not care about decreasing a stat if:

Your pokémon only knows special/physical attacks and you decrease the other one. For example, a Kyogre with Surf, Calm mind, Thunder and Ice beam won't be harmed if you decrease Attack.

Your pokémon is very slow and made to be slow, and you decrease Speed even more because it simply doesn't need Speed.

Your pokémon has a sky-high Defense or Special defense so it will still be high even though you decrease it.

Your pokémon has a pretty high Defense or Special defense but its only weaknesses are the other type, so you don't have any serious need for that defense stat.

Your pokémon has no need for something at all, like Shedinja has no need for Defense or Special defense since it will always faint in one hit anyway, so it's good to decrease them for Speed and Attack.

You may want to increase a stat, on the other hand, if:

Your pokémon is really just designed to use that particular stat so it's best to make as much use of it as possible.

Your pokémon has a bad stat that could use some increasing, but the other ones are distributed pretty fairly.

You're going to use this pokémon as a sweeper but it has low defenses and could use some more Speed.

You're going to use this pokémon as a sweeper and you want its Attack/Special attack to be even higher.